1. Field
Embodiments and implementations of the present invention relate generally to optics and more particularly to the field of fiber optics.
2. Brief Description of an Illustrative Environment and Related Art
Individual optical fibers (i.e., monofibers) and image conduits comprising multiple adjacent and ordered optical fibers (i.e., image bundles, that are either fused or flexible) are used in many industries for the purpose of conducting images from a remote location to a location proximate to an observer's eye or other detector. As is known in the field, such image bundles can conduct images of objects (i) with which the distal end of the optical monofiber or image bundle is in direct contact or (ii) for which an image of the object is focused onto the distal end of the monofiber or image bundle by a traditional converging lens, for example. In the latter case, the image receiving end of the monofiber, or plural fiber ends within an image bundle, are positioned to correspond to the focal plane of the lens or lenses used to focus an image onto the fiber end(s).
Efforts have been undertaken to create what are known as GRIN lenses and optical fibers exhibiting characteristics of GRIN lenses. The basic premise of a GRIN lens is that the degree to which an incident ray of light is bent at the interface between a first material and a second material or medium (and, thus, the ability to focus light) is a function of (i) the shape or contour of at least one of the materials or media and (ii) the relative refractive index of the first material with respect to that of the second material. If each of the first and second materials is of a uniform refractive index, then a focusing optical element (i.e., a lens) can be made only by varying the thickness of the optical element (i.e., contouring). As a corollary to this first observation, if one has as an objective to create a lens from a flat material having opposed parallel sides (e.g., “faces”) through which incident light rays are to pass, one can vary the index of refraction of the material from some maximum at the optical axis, for example, to some minimum, with the index of refraction decreasing in accordance with some non-linear function with radial displacement from the optical axis.
GRIN lenses are known and the general concept of a GRIN optical fiber is known. The known processes of forming GRIN optical fibers are, however, generally multi-staged and cumbersome. U.S. Pat. No. 5,673,353 represents a method of fabricating an optical fiber preform including, as stated in the abstract of the '353 patent, “ . . . a central core of a first material, a surrounding tube of a second material, and a deeply placed bonded layer integrally formed between the core and the tube preferably by a heat driven interdiffusion of the first and second materials. The deeply placed interface layer of the resulting preform structure exhibits material characteristics related to the interdiffused material characteristics of the rod and tube materials. The interdiffusion is preferably performed while supporting the combined rod and tube structure. The preform is rotated during heating to maintain the geometric symmetry of the preform and the interface layer. An encapsulating carrier is used to support the preform in all dimensions during heating.”
The specification of the '353 patent explains that an “ . . . advantage of the present invention is that it efficiently provides a highly controllable deeply placed bonded radial interface layer within an optical preform providing for a smooth transition of one or more properties between two materials. The controlled properties may include . . . index of refraction . . . ” The specification further emphasizes that, at least in most instances, and certainly the instances explained in any depth, the rotation of the preform during heating is about an axis orthogonal to any applied gravitational field. Although the representative processes disclosed in the '353 may be effective in achieving the stated objectives, they are undoubtedly cumbersome. Moreover, the processes of the '353 are apparently performed prior to the heating and drawing of any fiber.
Accordingly, there exists a need for a simplified method of fabricating a GRIN optical fiber in which the refractive-index-gradient profile is formed during the heating and vertical drawing of a core rod and a cladding tube, whether it be during the drawing of a single fiber or during the heating and drawing of a plurality of bundled fibers in order to fabricate a fused bundle comprising a plurality of GRIN optical fibers.